The invention relates generally to an electrophotographic process, and more particularly to an electrophotographic process for printing a number of duplicated copies with a single imagewise exposure of a document to be duplicated.
Nowadays various processes have been developed for forming a number of copies from an electrostatic latent image once formed on a photosensitive member.
(i) The latent image is developed with toner particle developer and only a part of a toned image is transferred onto a transfer member such as a record paper. Then the development and transfer steps are repeated successively to obtain a plurality of copies.
(ii) After developing the electrostatic latent image with toner particles a part of the toned image is transferred to a record medium. Then a plurality of copies are formed by successively repeating an electrification step, a uniform exposure step, a development step and a partially transferring step.
(iii) A development step and a transfer step are repeated without destroying the electrostatic latent image on the photosensitive member to form a plurality of copies.
(iv) Prior to an electrification the photosensitive member is subjected to an imagewise exposure and a plurality of duplicated copies are obtained with utilizing a persistent conductivity of photosensitive material.
(v) The original image of the document is temporarily stored and a plurality of copies are printed with utilizing the stored original image as a secondary document.
(vi) After forming on the photosensitive member a highly insulating resin layer corresponding to the document plurality of copies are formed by repeating the electrification, uniform exposure, development and transfer steps successively.
The known process (i) has been disclosed in, for instance, a Japanese Patent Application Publication No. 30,233/69 (corresponding to U.S. patent application Ser. Nos. 537,964 and 538,038 filed on Mar. 28, 1966) and a Japanese Patent Application Publication No. 7,789/71. In the process of the Japanese Patent Application Publication No. 30,233/69 the electrostatic charge latent image once formed on an insulating member is developed with toners and the toned image is partially transformed onto a transfer member while a transfer potential is applied across the insulating photosensitive member and the transfer member in such a polarity that the toners are attracted to the transfer member. After the transfer step the latent image on the insulating member is again developed with toners and the toned image is transferred to another transfer member while applying the transfer potential which is higher than that applied is the previous transfer step. In this manner the transfer potential is made higher progressively during the formation of a plurality of copies.
In the electrophotographic process described in the Japanese Patent Application Publication No. 7,789/71 there is provided an external electric field control member for generating an electric field having a direction opposite to that of the electrostatic field between the transfer member and the latent image retaining member. Across the control member and the retaining member is applied a voltage of about 500 to 900 volts to maintain an electric stress at a separating point below a breakdown value of air. Thus after the toned image has been transferred the electrostatic latent image is remained on the image retaining member without being distorted so as to form a plurality of copies from the electrostatic latent image once formed on the image retaining member.
However the known processes have disadvantages that the toned image formed on the photosensitive member is not sufficiently transferred, but only partially transferred and thus the density or concentration of the duplicated image is low. In order to increase the copy density the higher transfer potential may be applied, but in this case the electrostatic charge latent image might be destroyed and thus distortion and over development might occur in the duplicated copies. Further the number of copies obtained from the single electrostatic image is limited to only about ten and even if various supplemental measures are taken, only 20 to 30 copies could be formed. Moreover the known processes are liable to be affected by humidity and although the transfer potential is not so high, charges due to the transfer bias pass through the transfer member and the photosensitive member is undesirably charged. Thus the fog becomes noticeable on the copies in accordance with the progress of the multiple duplication.
A known process belonging to the process (ii) has been disclosed in a Japanese Patent Application Laid-Open Publication No. 8,730/72. In this process an electrostatic charge latent image once formed on a photosensitive member is developed with a liquid developer and a toned image is partially transferred to a transfer web under pressure. Next the photosensitive member having a residual toner image is charged and then is subjected to a uniform exposure so as to discharge the electrostatic charges on exposed areas. The residual toner image is again made in contact with the liquid developer and the developed image is again partially transferred to another transfer web under pressure. A number of copies can be printed by repeating the above steps.
However in this known process since the charging and exposing steps are effected after the transfer step and thus an amount of residual toner particles is less, the potential at imagewise dark areas is decreased by the uniform exposure although the photosensitive member has been charged to a given potential. Therefore it is very difficult to obtain the duplicated copies having sufficiently high density. Moreover the partially residual toner image is used as a secondary original document and thus a gray tone might be possibly changed during a formation of a plurality of copies.